Stroke volume is typically defined as the volume of blood that is ejected from one of the ventricles (e.g., the left ventricle) during contraction of the heart. The stroke volume of an individual may decrease during certain situations (e.g., blood loss during surgery) or as certain cardiovascular conditions develop. In some cases, stroke volume itself may be used to identify an emergency situation or a health condition. For example, when the stroke volume of a patient is very low, such as during excessive blood loss, the patient's body may be unable to circulate a sufficient amount of blood to the patient's organs and organ failure may occur. Accordingly, medical personnel typically desire an accurate method for determining stroke volume so that corrective action may be taken when the stroke volume falls below a designated value.
Various methods for determining stroke volume may be used, such as oesophageal Doppler monitoring, transpulmonary thermodilution, lithium indicator dilution, pulse power analysis, and pulse contour analysis. Pulse contour analysis may provide a less invasive method of determining stroke volume. In pulse contour analysis, the stroke volume is estimated by analyzing the pulse waves that are detected from a peripheral artery or from another anatomical location, such as a finger tip. Various models have been developed that estimate the stroke volume by analyzing the pulse waves. For example, one proposed model determines elements that estimate characteristic impedance, Windkessel compliance, and peripheral resistance of the patient. However, under certain circumstances (e.g., very high or low blood pressure, particular heart abnormalities, or during certain medical procedures), one or more of the above methods used to estimate stroke volume may have limitations that render the estimate unreliable.